This invention is related to the storage of data in a computer system, and more particularly to the emulation of direct access magnetic disk storage by means of fixed block architecture optical storage. More particularly, the invention concerns the conversion of a direct access magnetic storage data recording format to a direct access optical storage recording format to permit fixed block optical storage to emulate magnetic storage.
As is known, large computer installations employ direct access storage device (DASD) subsystems for fast, efficient, and economical storage of data in a device which records and retrieves data in response only to a reference of the data's location. The principal device which supports direct access storage is a disk. The most widely used disk technology in present-day computer installations employs a rigid magnetic disk on which data is written magnetically in a location format based upon parallel tracks in the form of concentric rings on each surface of the disk. Typically, a hard disk drive will contain one or more disks and will provide for the independent, direct accessing of each surface of each disk. In a multi-disk DASD, each surface of each disk is accessed by a respective head/actuator assembly so that identification of particular head identifies a particular disk surface. Within a multi-disk DASD, the set of all tracks of the same nominal circumference form a "cylinder". Thus, each track of a multi-disk DASD can be uniquely identified by a cylinder number (CC) and a head number (HH). A two-dimensional track address CC HH permits a track to be directly accessed by moving an identified head (HH) to the circumferential location of an identified cylinder (CC).
Fixed block architecture (FBA) optical storage is receiving increasing attention as a complement to, or a substitute for, magnetic DASD. FBA optical storage possesses a greater storage capacity than magnetic DASD in bits per storage area (areal density) and provides access to stored data at a speed that is somewhat slower than that provided by magnetic DASD. As is known, the use of magneto-optical technology supports a re-writable FBA optical storage which includes all of the storage features enjoyed by magnetic DASD.
Computer installation considerations may make it desirable to utilize re-writable FBA optical storage in place of magnetic DASD. However, the I/O (input/output) configuration of virtually all modern computer installations is based upon the magnetic DASD track format. FBA optical storage typically has only a single sequence of optical sectors in the form of a spiral "track" on each surface of each optical disk, implying only a single dimension for optical storage location format. In particular, data on an optical disk surface is addressed by only giving the sector number of the data. Therefore, in order to fully accommodate data storage in fixed-block optical storage for a modern computer installation, there is a need for mapping magnetic DASD track identification to FBA optical sector identification.
Mapping of one address form to another is taught in the prior art in the form of look-up tables. For example, U.S. Pat. No. 3,082,406 shows conversion of an external address location form to an internal address form in a data storage system by use of a look-up table. U.S. Pat. No. 4,947,367 teaches storage of magnetic tape formatted data on a WORM (write once, read many times) disk. In this patent, a look-up table is generated for addressing each tape, and the table includes a present WORM disk volume number storing a portion of the tape data as well as a next WORM disk volume number storing another part of the tape data. As is known, look-up tables are expensive in terms of storage space. Thus, as the capacity of DASD storage increases, track address emulation based upon look-up table address conversion would increase overhead by multiplying the storage space required for look-up tables, and might slow storage access due to table retrieval. Thus, the most desirable solution of the problem stated above should avoid the use of look-up tables to convert magnetic DASD track addresses to FBA optical sector addresses.